The coupling of light into a light-guide is a common requirement in optical systems employed in a range of applications including telecommunications, illumination, diagnostics, and solar energy collection. Light-guides generally include a core region of high refractive index surrounded by a cladding region of lower refractive index (which may be air or vacuum). Light rays that undergo total internal reflection at the interface between these two regions are trapped within the light-guide and can be routed along the light-guide to desired output locations. Light-guides are often fabricated as fibers or as planar slabs, but can also be formed in other geometries. Light is introduced into the light-guide at one or more coupling locations, where the light rays can be captured into guided modes of the light-guide. Often, light is focused onto the coupling location so that a high intensity of light can enter the guide at a small coupling location. For high-efficiency light coupling, precise alignment of the focused light to the coupling location is required. The need for precise alignment of an optical system adds considerable expense and complication to the assembly process. Furthermore, the system alignment must be re-established if the incoming light changes in position or direction.
One use of light-guides is in solar energy concentrators that gather light from an array of concentrating lenses or minors and direct it onto a receiving element, such as a photovoltaic cell. These light-guide concentrator designs have an advantage compared to traditional solar concentrating optics in that individual receivers need not be positioned at the focal point of each concentrating lens; instead, a single receiver can be positioned at the end of the light-guide to receive the collected light from many concentrating elements. In one prior art light-guide concentrator design, disclosed in U.S. Pat. No. 7,672,549, an array of concentrating elements is positioned above a light-guide. At the focal point of each concentrating element, a coupling site includes a mirrored facet in the light-guide that redirects the focused light so that it is captured by the light-guide. In order to couple this light into the light-guide without also incurring some loss of light captured from other lenses, the modal volume of the light-guide is increased at each coupling site. Similar prior art light-guide concentrator designs are disclosed in U.S. Pat. No. 7,817,885 and International Application No. PCT/US2009/034630, both of which describe concentrators featuring a sheet of concentrating lenses above a stepped or planar light-guide, with reflecting surfaces located at the focal points of the lenses in order to couple the focused light into the light-guide. A fourth prior art light-guide concentrator design, disclosed in International Application No. PCT/US2009/057567 and illustrated in FIG. 1a, also uses a planar light-guide 11 of constant modal volume and an array of concentrating elements 12. Coupling sites 14 are provided with a mechanism to reorient the concentrated light rays 13 so that they couple into guided modes of the light-guide. As illustrated in FIG. 1b, one design provided for a coupling site 14 is a sawtooth “fold” mirror 15 fabricated on the light-guide in a small area at the focus of each lens. This fold minor 15 is constructed with a 120° sawtooth design to deflect a normally incident cone of light rays 13 by +60° or −60° so that they will couple into the light-guide. The need for precise optical alignment in each of these concentrator systems complicates their manufacture. In cases where the light source is not stationary, for example in the collection of solar light, the systems are repositioned during operation by a mechanical tracker (not shown), which can be connected to or incorporated into the systems, in order to follow the motion of the light source (i.e., the sun, in the case of solar light collection). If the concentrator is not properly oriented with respect to the angle of incident light, the spot of focused light will no longer fall on the coupling minor 15 and therefore will not be captured by the light-guide.
Passive solar trackers have been designed using materials that move or change shape due to differential heating in the sun. Exemplary materials include evaporative liquids, bimetallic strips, and shape memory alloy. These systems are powered by incident sunlight and mechanically re-orient the entire solar energy system to face the sun.
The field of microfluidics investigates devices in which small amounts of liquid are controllably moved within confined volumes; the term “optofluidics” is sometimes used to describe such devices designed to achieve optical effects. International Application No. PCT/US2009/057567 describes the use of optofluidics to provide automatic solar tracking in a planar concentrator design. The document describes a scheme in which the electric field of concentrated light was used to trap nanoscale particles suspended in a fluid, thereby raising the refractive index of the fluid at the location of focused light.
A mechanism that can be used to manipulate fluids is the thermocapillary effect, in which a temperature gradient is imposed upon a fluidic system. The surface tension of a fluid (or the interfacial tension between two immiscible or partially miscible fluids) is dependent on temperature, so a temperature gradient across a fluid surface or interface will result in uneven surface tension that produces a net force and causes fluid movement. When a thermal gradient is imposed upon a layer of fluid, the spatially varying tension causes convection to occur within the layer, and in a thin fluid film these forces can result in local thinning or even rupture of the film. When a thermal gradient is applied to a droplet, unequal tension on opposite sides of the droplet can cause it to migrate. Using this technique, a droplet may be moved within an air or vapor environment, a gas bubble may be moved within a liquid environment, or a liquid droplet may be moved within an immiscible or partially miscible fluid. Droplets and vapor bubbles can be stably captured at hot or cold spots. The direction and speed of fluid movement is a function of the temperature gradient, the geometry of the system, the contact angle of the liquid or liquids upon the surface or surfaces, the viscosity of the liquid or liquids, and the sign and magnitude of the change in interfacial tension with temperature.
The thermocapillary effect has been exploited to control fluid flow in some microfluidic devices. In various experiments, the temperature gradient generally was obtained either by using resistive heating elements or by shining light from a laser or high-intensity lamp onto an absorbing element or fluid. Large temperature gradients resulted in rapid movement of the fluid interface, and convection currents were generated in the bulk liquid or liquids.